Keywords

11.1 Introduction

Urban air pollution is one of the important environmental concerns worldwide. Atmospheric aerosols as well as gaseous pollutants such as SO2, NO2, CO, O3, etc. pose severe health effects both for humans and plants (Maatoug 2010). Certain nonradioactive metals such as mercury (Hg), zinc (Zn), and arsenic (As) and radioactive metals such as strontium (Sr) and cesium (Cs) also have toxic effects for health and the environment. Plants are considered a very good responder for selected air pollutants. The science of air pollutant monitoring through plants is known as biomonitoring. In its historical outlook, biomonitoring is a very old technique to study air pollution. Long ago in the seventeenth century, John Evelyn, in his book Fumifugium which was published in 1661, first time mentioned about air pollution damage to vegetation. In the nineteenth century, Nylander (1866) used lichen population as air pollution indicator. During the past century in 1958, biological indicators were used for the same purpose in the Los Angeles basin in the USA. Later, Heck (1966) and Heggestad and Darley (1969) reported air pollution effects on the tobacco plants in California. In the Netherlands, Van (1969) used plant indicators to understand the effect of SO2 and HF. However, biomonitoring received attention after the work of Schonbeck et al. (1970) who reported certain unique characteristics of biological indicators. These physiological and biological features of plants gather information related to impact of air pollution which cannot be accessed through chemical methods of air pollution monitoring.

In biomonitoring through plants, the changes occurring in plant morphology and physiology or biochemical changes are used as the indicators of pollution impact. Generally, we use passive and active methods to observe the changes and response of plants. In the passive method, plant growth is seen as a natural process of development. In the active method, the presence of different air pollutants is determined by plantation of an experimental plant for its response and genotype at a location in which plant response is assessed indirectly by recording biochemical and physiological changes in the plant. For example, parameters such as foliar injuries, stomatal pore size, chlorophyll content, etc. are used to observe the response of a bioindicator against air pollutants. Bioindicators can reveal the impact and the cumulative effects of different pollutants. The statement of Tingey (1989) justifies the importance of a bioindicator in an appropriate manner. He says “There is no better indicator of the status of a species or a system than a species or system itself.” The biomonitor can be used to monitor the effects of air pollutants with the changes in the temporal and spatial variations. However, while using biomonitoring, their standardization method is very sensitive especially for the development of air quality standards to prevent harmful effects on the plant health and ecosystem.

Since plants interact with air and air pollutants, their role becomes crucial because such interactions can alter the atmospheric environment including local meteorology and pollutant levels. Hence, the plants can be used as a phytoremediator of air pollutants which is a very useful tool for purifying air in urban areas. Both gaseous and particulate pollutants can be removed by the plants. It has been found that some plant species are useful to monitor a single pollutant as they are sensitive to a specific pollutant, while certain species are useful to monitor mixtures of pollutants.

11.1.1 Significance of Biomonitoring

The biomonitoring of air pollution by plants offers some important results from different abilities:

  1. 1.

    Biomonitoring provides very important information about the effects of pollution as these effects are exhibited as visible injuries in sensitive plants while as an accumulated pollutant in less sensitive species. Even tolerant plants also accumulate pollutants which can be used as indicators.

  2. 2.

    Sometimes, very low concentrations of air pollution are difficult to measure with chemical and physical methods, but plants can accumulate such pollutants to a level which can easily be analyzed.

  3. 3.

    Some plants show an integrated response against the pollution and climate; the risk potential of particular pollutants or the mixture of pollutants can be estimated more realistically.

  4. 4.

    Biomonitoring becomes highly useful in monitoring different levels of the organization of plants ranging from the individual plant (or even single cell to leaf) to the plant cluster and up to the ecosystem level e.g. during the shift in species composition at the community level is a result of an integration of different factors over a longer period experienced by plant species; response is estimated more realistically by a biomonitor rather than any physicochemical method.

  5. 5.

    Unlike physicochemical monitoring, biomonitoring offers monitoring of a large-scale pattern of pollutant distribution and temporal changes without involving maintenance cost. Many of these attributes render biomonitoring as being suitable for both developing and developed countries.

11.1.2 Definition of Biomonitoring and Terminology

Biomonitoring involves several different terminologies, such as bioindicators, biosensors, bioaccumulators, and biointegrators, which have been described below:

  • Bioindicators: The individual plants which have visible symptoms, e.g., chlorosis, necrosis, and disturbed physiology, are known as bioindicators. These provide information about the quality of the environmental conditions.

  • Biosensors: These are those plants which respond to the presence of air pollutants; these are also known as biomarkers. These plants have non-visible effects. Detection of effects at molecular, cellular, biochemical, or physiological levels needs microscopic and physiological techniques, as well as biochemical analysis.

  • Bioaccumulators: These plants which can accumulate air pollutants such as aerosols, dust, and gaseous molecules into their tissues are called bioaccumulators. These are also known as accumulative indicators.

  • Ecological indicators: It is a slightly different category than the above described. The concept of an ecological indicator is related to the population and community loss of plants. Plants related to these categories are also known as biointegrators. Ecological indicators highlight the changes in composition of the species in an ecosystem along with their appearance and disappearance and the variation in their density in a given area.

11.2 Biomonitoring by the Deposition/Accumulation of Air Pollutants in Plant Tissue

Various methods of biomonitoring apply the whole or part of an organism to measure exposure of a plant as well as accumulation of a pollutant. Based on the specific response of plants against pollutants, it can be classified as a good bioindicator, while tolerant species are generally used as a bioaccumulator. If the rate of clearance of a pollutant from the organism is known, we can do direct quantitative assessment of exposure by analyzing plant tissues. Various known methods have been used to assess the effects of pollutants such as:

11.2.1 Biomonitoring of Nitrogen Deposition

Biomonitoring has been found a very useful technique for the measurement of foliar nitrogen, atmospheric NH3, and total nitrogen deposition. Bobbink et al. (1993) measured total tissue nitrogen (N) for many years in all types of plant tissues to find out atmospheric nitrogen deposition. Pitcairn et al. (1998) found this method of assessment very useful for tissue nitrogen content. They observed that the foliar nitrogen concentrations decreased with increasing distance from the livestock buildings. These workers noticed that there was a close relationship between foliar N, atmospheric NH3 concentrations, and total N deposition for each selected moss, herb, and tree species.

11.2.2 Biomonitoring for Sulfur Deposition

The plants absorb sulfur dioxide (SO2) from the atmosphere primarily through their leaves (Laisk et al. 1988). This can be an alternative source for normal growth in the situations where soil S is low. However, excess sulfur from the atmosphere might have an adverse impact on plant growth when S in soil is present in an adequate quantity. Manninen and Huttunen (1995) found that young Scots pine needles accumulated S proportionally to the ambient SO2 load. Haapala et al. (1996) found that S content in pine needles was observed decreasing with the increasing distance from the known pollutant source. Sulfur content of needle tissues of Sitka and Norway spruce was observed to be higher corresponding to higher dry deposition of sulfur (Innes 1995).

11.2.3 Biomonitoring for Dust Deposition

Biomonitoring can be used for estimating dust deposition. Gupta et al. (2015b, c) have reported the average dustfall deposition fluxes on arjun leaves at two different sites. At the urban site, they recorded dust fluxes at 57 ± 3, 120 ± 5, and 117 ± 7 mg/m2/day during monsoon, winter, and summer seasons, respectively, while at the industrial site, these workers found dustfall fluxes at 151 ± 4, 286 ± 6, and 259 ± 8 mg/m2/day during monsoon, winter, and summer seasons, respectively.

11.2.4 Biomonitoring of the Chemical Component of Dust

Once collected on the leaves, the dustfall can be analyzed for the ionic content process in a water-soluble fraction. Fluxes of ions such as SO4 2− and NO3 − can be estimated as discussed below:

11.2.4.1 Dustfall Fluxes of SO4 2- and NO3 − on Foliar Surfaces

As mentioned earlier, biomonitoring of ionic species in the aqueous extract of dustfall recorded higher fluxes of (SO4 2- + NO3 −) as reported by Gupta et al. 2015a at an industrial site in Delhi which have been attributed to the local emissions. Chemical analysis of dustfall can provide information about the deposition rates; covariation of ionic species is important to know their sources and transformations. The fluxes of both SO4 2- and NO3 − were much lower at the urban background site as compared to the industrial site (Table 11.1). Table 11.1 gives the dustfall fluxes SO4 2- and NO3 −. The deposition of both the ionic species is in accordance with the ambient concentration of SO4 2- and SO2 for SO4 2- fluxes while with NO3 − and NO2 for NO3 − fluxes, respectively.

Table 11.1 Dustfall fluxes of SO4 2- and NO3 − on arjun foliar surface and SO4 2- and NO3 − aerosol and ambient SO2 and NO2 concentrations at industrial and urban sites
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Their seasonal levels also varied according to the abundance of respective aerosols and gaseous species in ambient air. As given in Table 11.1, SO4 2- fluxes were much higher than the NO3 − fluxes. The reasons for lower NO3 − fluxes may be due to formation of nitrate (NO3 −) or nitrite (NO2 −) after NO2 entry through the epidermal layer and the substomatal chamber reaching the mesophyll cell and reacting with the mesophyllic cell wall of the ascorbic acid (Ramge et al. 1993). In order to fight against stress and to show increased tolerance, ascorbic acid production was higher at the industrial site than that of the urban background site which may be due to enhanced oxidative stress for activity in situ. In this process, reaction to ascorbic acid plays an important role. Due to this reason, the plants having higher levels of ascorbic acid in their leaves show a higher rate of NO2 uptake (Teklemariam and Sparks 2006) which may result in higher NO3 − and NO2 − accumulation in cells producing greater acidity in the leaves. As reported by Gupta et al. (2015a, b, c), higher fluxes of SO4 2-may be due to the dry deposited SO2 and its oxidation of SO4 2- in a polluted and dusty atmosphere (He et al. 2014; Kulshrestha et al. 2003; Seinfeld and Pandis 1998). In addition, SO2 adsorption onto the dust particles (already settled on the surface of leaves) can also react in the formation of CaSO4 (Gupta et al. 2015a). CaCO3 rich soil dust in India significantly scavenges atmospheric SO2 in the form of CaSO4 (Kulshrestha et al. 2003, Kulshrestha 2013). Furthermore, the oxidation of SO2 is catalyzed by NO2 forming SO4 2- formation (He et al. 2014). High SO4 2- formation due to NO3 − present on foliar surface which enhances the hygroscopicity of mineral particles on the leaves increases the possibility of SO2 oxidation. As reported by Singh (2014), gas/aerosol partitioning coefficients were favoring NO2 existence in the gas phase as compared to the aerosol phase (NO3 −) with respect to SO2/SO4 2- existence.

11.2.5 Biomonitoring of Deposition of Heavy Metals

Mosses and lichens have been commonly used as biomonitoring species for metal pollution. Some workers have found that Lolium multiflorum ssp. italicum cv. “Lemma” is a very good biomonitor for trace elements, fluoride, sulfur, and organics at urban sites in Europe as this species grows faster and provides reliable and quick information (Rodriguez et al. 2010; Klumpp et al. 2009). However, higher plant species are also good biomonitors for selected trace elements.

Heavy metals, such as Pb, Zn, and Cu, were found to be accumulated in plant leaves of two urban plant species, viz., plane and cypress, which were used for air quality assessment in Tiaret city in Algeria (Maatoug 2010). Results showed that the ratio of fresh weight to dry weight (FM/DM) of leaves is affected by trace metal concentrations. The study showed that FM/DM ratio had an inverse relationship with metal concentrations. Experimentally observed lower ratios were attributed to poor air quality near the site of investigation.

11.2.6 Biomonitoring of Gaseous Pollutants Due To Changes Through Physiological and Biochemical Changes in Plants

11.2.6.1 Monitoring of Crops, Vegetables, and Trees Damaged by Air Pollution

Air pollution may damage the crops, vegetables, and trees in various manners. Agricultural productivity in urban areas has been found low due to high emissions and concentrations of SO2, NO2, and O3 (Rai et al. 2011; Nandy et al. 2014). Air pollution effects can be seen as visible symptoms where the plant has acute injury. Even the growth response effect in some vegetables by air pollution has been reported by Ghouse and Khan 1984. Ashmore and Marshall (1999) found that the agriculture crop was severely injured by ozone. Dhir et al. (1999) found that the crop of Achyranthes aspera Linn. exhibited air pollution effects on morphology and some physiology of the plant. It was also reported that air pollution affects the growth and reproductive behavior of mustard plants (Saquib and Khan 1999).

Coal-smoke pollution was found to affect the stomatal conductance, photosynthetic rate, and pigment content in Ruellia tuberosa (Nighat and Mahmooduzzafar 2000). Acute injury was noticed due to SO2 showing bifacial chlorosis and marginal and interveinal necrosis on leaves (Legge and Krupa 2002). Evidence of oxidative stress was found to affect the plant physiology which exhibited spots on barley leaves during spring and winter seasons (Wu and Tiedmann 2002). Elevated ozone exposure was found to affect the photosynthesis, growth, and productivity of soybean (Morgan et al. 2003). Severe effects of O3 may result in death of plant organs. It also affects plant growth leading to a low yield (Ashmore 2005).

Air pollution effect on yield and quality of mung beans has been reported by Agrawal et al. (2006) for peri-urban areas of Varanasi in India. Ambient air pollution was found to reduce the yield by 43 %, 39 %, and 18 % in three wheat cultivars, respectively, at mean concentrations of 70, 28, and 15 ppb of O3, NO2, and SO2, respectively, during different seasons at Lahore in Pakistan (Wahid 2006). At ambient levels of SO2 (7.8 ppb), NO2 (40.6 ppb), and O3 (42.1 ppb), Rai et al. (2007) noted 20.7 % reduction in wheat yield in Varanasi, India. According to Heath (2008), O3 generates ROS which causes damage to plants. The initial site of injury is the plasma membrane due to which permeability and fluidity of cells are changed. During changing of climate, elevated levels of CO2 and O3 affect the rice production (Ainsworth 2008). Higher air pollution load (SO2 6.5 ppb and NO2 9 ppb) caused reduction in wheat and mustard crop yield in Haridwar, India, as reported by Chauhan and Joshi (2010).

Gupta et al. (2015a, b, c) have reported a significant impact on biochemical constituents such as chlorophyll, carotenoids and soluble sugar, ascorbic acid, and proline on Arjun (Terminalia arjuna) and Morus (Morus alba) plants due to the urban dust deposition. They reported these changes in biochemical constituents in accordance with the stress level at two sites. The biochemical changes were noticed more prominently at the industrial site as compared to the urban background site due the greater oxidative stress caused due to higher accumulation of dustfall, particulate SO4 2-, etc. at the industrial site.

11.2.6.2 Biomonitoring of Dust Particles Through Foliar Surface Morphology Observation

As mentioned earlier, both gaseous and particulate pollutants affect physiological and morphological characteristics of plants (Rai et al. 2010). According to Kim et al. (2000) and Chaturvedi et al. (2013), sedimentation of coarse particles has more impact on the upper surfaces of leaves. Deposition of dust particles has been noticed more at the industrial site than the residential site (Gupta et al. 2015a, b, c), whereas the finer particles have more impact on lower surfaces of leaves (Fowler et al. 1989; Beckett et al. 2000). Larger-sized particles pile up on the foliar surfaces, while the smaller particles enter through the stomata affecting the photosynthesis, gaseous exchange, and water retention further affecting the growth and yield of plants (Tomasevic and Anicic 2010; Rai et al. 2010). Dust particle deposition can cause reduced stomatal diffusive resistance and an increased leaf temperature (Fluckinger et al. 1979). More damage in the upper and lower epidermis and reduction in palisade parenchyma cells have been reported by Stevovic et al. (2010) at a polluted site as compared to a nonpolluted site. They found that erosion of the epicuticular wax and cuticle rupture were more frequent on the adaxial side, whereas loss of sinuosity on the anticlinal wall of the epidermal cells and stomatal deformity and obstruction were seen on the abaxial side of the leaves (Rocha et al. 2014).

The foliar surface morphological analysis verifies the deposition of industrial and road dust which effects damage in the cuticle and epidermal layer at adaxial site and at abaxial site damages in guard cells and stomatal pores. The abrasive effect of dust deposition restructures the cuticle and affects the epicuticular wax. Dust depositions also rupture epidermal hairs along with swollen guard cells (Gupta et al. 2015a, b, c). Also, dust deposition causes changes in the size of the stomata. Gupta et al. 2015a, b, c reported that the size of the stomatal pore was smaller at the industrial site (9.52 × 2.43 μm), whereas it was larger (13.5 × 4.74 μm) at the urban site. Even SO2 and NO2 cause wax degradation and surface needle erosion (Grodzinska-Jurczak and SzarekLukaszewska 1999). The length (L), breadth (B), and L/B ratio of leaves are significantly changing according to the automobile air pollution scenario at the site (Nandy et al. 2014). They observed pigmentation, chlorosis, and necrosis symptoms in roadside plant species such as Alstonia scholaris, Neolamarckia cadamba, Ficus benghalensis, and Ficus religiosa.

11.3 Phytoremediation of Atmospheric Pollutants

The process of cleaning up the environment using plants is called phytoremediation. Phytoremediation has its origin from the Greek word phyto, which means “plant,” and the Latin word remediare, which means “to remedy.” This term is usually used to describe the removal of contaminants by using plants from any system, i.e., air, water, soil, etc. (Chhotu and Fulekar 2009). When it comes to remediation of pollutants, plants have always been considered as effective removers of pollutants for soil, water, and air. Plant-based remediation is highly low cost and is efficient in the removal of pollutants/contaminants. Interestingly, as transgenic are being tested in the field and the associated risks assessed, their use appears to be more accepted and less regulated than has been the case for transgenic crops. Approaches such as molecular, biochemical, and physiological are being applied to identify the plant mechanisms by which they accumulate gaseous, particulate matter and metal aerosols. They have also exerted effort to use transgenic plants for phytoremediation in order to find out genes responsible for this unique biological property. During the two decades, phytoremediation work has received great attention from researchers worldwide.

Among various plants, trees are having superiority for air filtration as compared to shrubs and grasses. Due to this probability, trees have been termed as the “lungs of cities.” Removal of air pollution increases with leaf surface area, and hence evergreen trees are considered as highly effective pollution filters as the evergreen trees have a large surface area and year-round coverage. Therefore, trees tend to be better filters than shrubs and grasses. Several species of ornamental shrubs as well as herbaceous plants have been identified as phytoremediators to purify the indoor and outdoor air quality.

11.3.1 Principles of Phytoremediation

Phytoremediation is based on rhizodegradation, phytostabilization, phytofiltration, phytoextraction, phytodegradation, and phytovolatilization (Morikawa and Erkin 2003) (Fig. 11.1).

Fig. 11.1
figure 1

Different parts of plants are responsible for different processes of phytoremediation

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Phytostabilization: it is the process in which contaminants are sequestrated in the root zone.

Rhizodegradation: it is the process of degradation of pollutants by root secretions and rhizospheric microorganisms. The rhizosphere has more densities of microorganisms in a narrow region of 1–3 mm from root surface than in bulk soil.

Phytoextraction: it involves extraction of pollutants from soil and their translocation from the roots to shoots.

Phytodegradation: it is the process of degrading the pollutants which entered from the soil, water, or air to the plant.

Phytovolatilization: in this process, pollutants are volatiled out of the stoma where they are degraded by hydroxyl radicals.

Phytofiltration: it is the process of removal of particulate matters through the surface of plants.

11.3.2 Two-Compartment Model for Phytoremediation

In order to explain atmosphere-foliage bioaccumulation, a two-compartment model has been used (Keymeulen et al. 1995; Mackay et al. 2006). This model considers a relatively fast initial uptake followed by a period of a slower uptake. In this model, the leaf is divided into two compartments. The first compartment has the nonliving plant cuticle. In the first compartment, occurrence of physicochemical sorption of airborne lipophilic compounds takes place, while the second compartment has the leaf interior. In the second compartment, sorption and metabolism of organic compounds take place.

The final fate and disposition of organic pollutants (including xenobiotic compounds) within plants have been explained by using the “green liver” model. The process of detoxification metabolism of foreign compounds in plants is very similar to the metabolism of xenobiotic compounds in the human liver system, and, hence, the term “green liver” is used in this model (Burken et al. 2000).

11.3.3 Properties of Air Phytoremediators: Ornamental Plants and Woody Trees

The relevant criteria for the ornamental and woody trees to be phytoremediators for air pollution are they should be evergreen trees having broad leaves, very rough barks, low water requirement, ecological compatibility, high capacity for absorption of pollutants, minimum care needs, agro-climatic suitability, and good height and spread. The canopy of these plants is suitable for an aesthetic effect. Besides their attractive flower color and conspicuous foliage, tolerance and dust removal capacity also make these trees effective phytoremediators of air pollution (Brethour et al. 2007; Kumar et al. 2013).

Generally, ornamental and woody trees are effectively used as phytoremediator tools for reducing air pollution (Zhai 2011). These types of plants are part of the greenbelt in urban areas and can be used for phytoremediation of air pollutants. Such plants remediate air pollution via absorption and degradation of urban air pollutants resulting in reduced levels of air pollutants (Brown 1997; Yoneyama et al. 2002; Burchett et al. 2008). In general, trees are ideal in the remediation of heavy metals because they can withstand even after an accumulation of a higher amount of pollutants (Shah and Nongkynrih 2007). Some fast-growing trees, e.g., eucalyptus, pine, and poplar, are found to be very effective in air pollution removal. Also hardwood trees, e.g., rosewood and mahogany, are used for remediation of polluted air. According to a study, around 711,000 metric tons of air pollution is removed by urban trees every year in the USA (Nowak et al. 2006). Table 11.2 gives the estimated removal of specific pollutants by trees (Nowak et al. 2006).

Table 11.2 Average air pollution removal and value for all urban trees in the USA
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11.3.4 Examples of Phytoremediation of Various Air Pollutants

11.3.4.1 Particulate Matter

Vegetation canopies effectively protect and remediate the dust of the atmosphere this process is called phytofiltration. According to the studies, an 8 meter wide greenbelt may reduce two to three times of dustfall (Novoderzhikina et al. 1966). Phytofiltration becomes more efficient if the vegetation is selected based on suitable morphological characteristics such as leaf orientation on the main axis, leaf size and shape, leaf surface nature, the presence or absence of trichomes on the leaf, and wax deposition on the leaf which help in capturing atmospheric dust.

11.3.4.2 Heavy Metals

As trees are good bioaccumulators, they can remove and store a huge amount of heavy metals. In a study, concentrations of five metals (Cu, Cd, Cr, Pb, and Ni) were determined in tree leaves. These were collected from 13 different areas in Greece. Geographical distribution patterns were investigated, and factors affecting toxic element accumulation in trees were discussed (Sawidis et al. 2012). The researcher found that the order of average heavy metal content in the tree leaves was Cu>Pb>Ni>Cr>Cd. High Cr, Cu, and Ni are found in Citrus aurantium leaves which are probably because of stomatal uptake. The conifer tree Pinus brutia which has a rough leaf surface showed high accumulation of Cd and Pb. Broad-leaved Olea europaea which has a thick waxy cuticle forms a smooth sheet due to which entry of metals is restricted through the epidermis. The trichomes also act as protective screen factors to keep away air pollutants. Sometimes, even the presence of a certain metal within the foliar cells can reduce the uptake or toxicity of other metals (Sawidis et al. 2012).

11.3.4.3 Inorganic Pollutants

11.3.4.3.1 NOX and particulate N

Air pollution of NOX is caused by vehicular exhaust, industries, and biomass burning. NOX is one of the precursors of photochemical reactions. NOX can deposit on plant leaves through wet or dry deposition. During the dry deposition of both gases and partciles on the leaves surafecs NH4 + and NO3 - particulates are also deposited (Davidson and Wu 1990; Bobbink et al. 1992).Most of NO2 enters into plants through stomata where it is metabolized to organic compounds such as amino acid. In this process, plant enzymes like nitrate and nitrite reductase or glutamine synthetase play an important role (Davies, 1986; Allen, 1988).

11.3.4.3.2 SO2

Most of SO2 in the atmosphere is emitted by fossil fuel combustion. Atmospheric SO2 enters into plants through the stoma where it is utilized as nutrient. SO2 is changed into SO4 2−/SO3 2− in cell walls which finally produce cysteine or other organic compounds; if the SO2 concentration is beyond limits, it causes severe acidity for plants and generates stress conditions.

11.3.4.4 Organic Pollutants

Plants have been reported to reduce the ambient organic pollutants such as formaldehyde, benzene, and toluene. Formaldehyde is a ubiquitous air pollutant removed by plant leaves as reported by NASA’s research in the 1980s. It is found that spider plants (Chlorophytum comosum L.) metabolize formaldehyde in shoots in the form of organic acids, free sugars, amino acids, and lipids (Giese et al. 1994). Godish and Guindon (1989) reported that up to 50 % of formaldehyde is removed by spider plants.

Benzene and toluene can also be removed from the ambient air by plants (Ugrekhelidze 1997). In a test chamber study, Opuntia microdasys was found to remove 2 ppm of toluene from air completely in 55 h. D. deremensis took 120 h to remove toluene from the air in the test chambers. In a study reported by Mosaddegh et al. (2014), toluene in test chambers was removed at 1.47 and 0.67 mg/m3 day1 rate for Opuntia microdasys and D. deremensis, respectively.

Yang et al. (2009) found that Asparagus densiflorus, Hedera helix, Hemigraphis alternata, and Hoya carnosa had the greatest removal efficiencies among 28 species tested. He found that Tradescantia pallida was superior in removing benzene, toluene, TCE, and α-pinene VOCs. Ficus benjamina was found as an effective remover of octane and α-pinene from air.

Among the tested plants, Chlorophytum comosum was superior in removing HCHO and SOx from the air at 1830 μg day−1 and 2120 μg day−1 rate, whereas Spathiphyllum wallisii for NOx was effective at 3200 μg day−1 rate (El-Sadek et al. 2012). Based on the air pollution tolerance index (APTI), Nugrahani et al. (2012) have identified the species of landscape ornamental and herbaceous shrubs which can be used as bioindicators of urban air pollution. These are Mussaenda philippica, Heliconia psittacorum, Ipomoea batatas, Jatropha pandurifolia, Bougainvillea sp., Hymenocallis speciosa, Codiaeum variegatum, Cordyline terminalis, Canna indica, and Sansevieria trifasciata.

Prescod (1990) suggested that orchids can be used during the daylight hours for efficient removal of multiple pollutants. According to his report, carbon dioxide and xylene (Anonymous 2007) are also effectively removed by plants during the night due to a unique metabolic process of orchids (and bromeliads) because their stomata (Ibrahim et al. 2008) open during nighttime.

11.4 Applications of Genetic Engineering for Phytoremediation

11.4.1 Transgenic Plants for NO2 and VOC Pollution Control

Genetically engineered plants can provide adequate sinks of air pollutants is an important to develop. These plants are termed as “wonder plants” which can clean up the specific pollutants from the atmosphere. Primary metabolism of nitrate involves enzymes such as nitrate reductase (NR), nitrite reductase (NiR), and glutamine synthetase (GS) which play a key role in NO2-nitrogen metabolism in the plants. All genes for NR, NiR, and GS are nuclear encoded. Takahashi et al. (2001) found positive correlations for NiR gene overexpression and NO2 assimilation in transgenic Arabidopsis plants. Doty et al. (2007) developed transgenic poplar (Populus tremula × Populus alba) plants with greatly increased rates of metabolism for the removal of VOC pollutants from the atmosphere through the overexpression of the cytochrome P450 2E1 enzyme.

11.4.2 Gas-Gas-Converting Plants That Convert Nitrogen Dioxide to Gaseous Nitrogen

Genetically engineered plants can convert NO2 to N2O or to N2 (gas-gas-converting plants). Generally, NO3 − or (NO2 −) is converted to N2 or N2O in denitrification process through denitrifier bacteria and fungi, respectively (Shoun et al. 1992; Zumft 1997). Goshima et al. (1999) have demonstrated transgenic tobacco showed reduced NiR activity by the expression of NiR cDNA in an antisense orientation and emitted N2O when fed with 15N-labeled nitrate and nitrite (Vaucheret et al. 1992; Takahashi et al. 2001). This invention of a gas-gas-converting plant that converts NO2, via N2O, to N2 will be a highly useful approach for air pollutant control.

11.5 Conclusion

Biomonitoring has become a complement of traditional techniques of air quality measurements. The identification of pollution within sensitive organisms also allows detection and degradation of air quality before the biota is severely affected. In this context, sensitive plants can be considered as real bioindicators of pollution stress generated by gases as well as particulate matter. Phytoremediation of air pollution is proved to be useful in reducing air, water, and soil pollutants. This technique is cost effective as compared to physicochemical methods. Since it is a natural phenomenon, it does not require energy supply. Plantation increases the aesthetic value of the environment too. This approach needs to be included in urban town planning as a mandatory component which will help in improving air quality of megacities in the future.